Process for removal of hydrogen fluoride from nonaqueous fluids and porous body for use in such process



Patented Aug. 26, 1947 PROCESS FOR REMOVAL OF HYDROGEN FLUORIDE FROMNONAQUEOUS FLUIDS AND POROUS BODY FOR USE IN soon PROCESS Kenneth E.Long, South Euclid, and Harold W. Cromer, Cleveland, Ohio, assignors toThe Harshaw Chemical Company, Elyria, Ohio, a

corporation of Ohio No Drawing. Originalapplication September 19, 1944,Serial No. 554,854. Divided and this application June 2, 1945, SerialNo. 597,378

3 Claims.

This invention relates to removal of hydro e fluoride from non-aqueousfluids inert to sodium fluoride.

The object of the invention, broadly stated, is to provide a superiorprocess for removal of hydrogen fluoride from such fluids, such processinvolving the use of a novel absorbent body of sodium fluoride. Morelimited objects of the invention are: to provide a suitable porous bodyof sodium fluoride capable, under the conditions of the process, ofbeing used repeatedly for absorption of HF from such fluids; to providea method of making such porous bodies having the indicatedcharacteristics; and to provide a commercially practicable process basedupon the use of such porous bodies for removing HF, especially smallamounts thereof from such fluids as air, nitrogen, petroleum gases,hydrocarbon liquids, etc. Further and more limited objects of theinvention will become apparent as the description proceeds.

The porous body and process of makin it e We have discovered that aporous body suitable for our purpose can be produced by making a pelletof sodium bifiuoride (NaFHF) and then heating it to decompose the NaFHFto NaF and HF. The HF escapes as a gas leaving a skeleton of NaF whichis microporous and highly absorptive with respect to HF under theconditions of the process to be described. Sodium bifiuoride in finelydivided crystal form, commercial crystal sodium bifiuoride screenedthrough a 35 mesh screen to remove all coarse particles or aggregates,and containing a great variety of particle sizes down to and below 100mesh, is pelleted in a Stokes pelleting machine or any other pelletingmachine without the admixture of any lubricant. The pelleting pressurepreferably is regulated to give a pellet of apparent density of 1.7 to1.9, as

2 example, 100 C. to 600 C'., but time and economy considerationsindicate the narrower range as the better practice. At whatevertemperature is selected and under whatever pressure conditions are foundpreferable (atmosphere pressure is most practical), the heating iscontinued until most of the HF is removed, preferably until the apparentdensity of the pellets is from 75% to 85% of the apparent density of thebifiuoride pellets heated. (This would result in sodium fluoride,pellets of apparent density from 1.3 to 1.5.) Since; the absolutedensity of NaF is given in the handbooks as 2.79, it can be seen thatthe resultin pellets are quite porous.

Use of porous NaF bodies for absorption of HF It is known that NaF canbe used to absorb HF,

- but to the best of our knowledge, no one has heretofore so controlledthe conditions as to make possible the use of a porous body of NaFwithout damage to such porous body. We have discovered determined fromcalculated volume as found by micrometer measurements on single pelletsand weights of the same pe1lets,average values from numerousdeterminations being used. The pellets may be made in sizes such as /8"D. by /s" long, e D. by 3% long, or other suitable size, or in the formof much larger briquettes which would then be crushed to granular form.It is expected that the smaller pellets will be commercially preferable.

The pellets of NaFHF are heated to drive ofi the HF or most of it, thusleaving a porous pellet of NaF. The temperature of heating for remova1of HF is preferably from 300 C. to 500 C. It is, of course, possible toemploy other temperatures, for

that if the temperature is held above C., the porous NaF bodies abovedescribed will not swell or disintegrate whereas below that temperaturethere is an increasing tendency to swelling, disintegration and evenliquefaction when the quantity of HF absorbed is too great. Above 85 0.,the absorption of HF ceases before there is any damage to the porousbody. In order to insure safe operation, we prefer to pass the fluidfrom which the HF is to be removed over the pellets at a temperature notlower than G. Control may then be allowed to vary 10 centigrade degreesup or down without danger. Good results may be had at C. but if thetemperature goes higher than 125 C., the capacity for absorptiondiminishes until results are unsatisfactory.

Operating at 100 C., for instance, there may remain more HF thandesirable, especially where nearly perfect removal of HE is required. Insuch cases, we may pass the air, or other fluid from which HF is to beremoved, through a series of absorption chambers maintained at 100 C.(and containing the porous bodies of NaF), and then through one or morechambers containing such bodies and maintained at temperatures below 85C. For example, such chambers may be at any temperature down to minus 80C. In such case, damage to the pellets is prevented because the amountsof HF to be removed are so small that the amount absorbed by any pelletin a normal run is too small to cause swelling. In other words, thechambers which operate above 85 C. will require reconditioning beforethe low temperature unit being used to absorb traces of HF amasse haveabsorbed an amount which could cause damage to the-:pelletsa, Thenumberof absorbing units operatinglabove 85? C. isso selectedas to producethis result.

When the NaF pellets have absorbed HF to;

the extent that the absorption rate has become unsatisfactory, they maybe regenerated-inth'ei same way they were produced; that'is, vby heatingbetween 100 C. and 600 C., preferably from 300 C. to 500 C. Both in theoriginal production of the pellets and in the regeneration, it isdesirable to have a gas sweepingrover:thepellets to remove the HF as itis evolved; TheHF may" be recovered from the gas in which it is thusentrained, but its recovery is-ioptionals A'lsatisgfactory system ofequipment would includea' series of heated towers and one ormore-ref-riger--- ated towers, each filled with the pellets. During oneportion of th'e cycle a fluid, for example air, containing. HIE wouldhe. passedthrough the seriesof towers, firsttlirough' the heated towers,thenthrough the final refrigerated tower or tow ers. v Duringtheregeneration phase; preheated air would be passed through the train,heating being continued in the case of the heated :tow-

ersan'd refrigeration, being discontinued in thecases of therefrigerated tower or towers. the air (orhother gas) is preheatedsuflicie'ntly, the .towers need not be heated, and on the otherhand-,1preheatingrcanrbe eliminated if heating of the vtowers isincreased-so that thetemperature ing ,mechanically-strong bodies-of= a'sizetoo' larg 4 to pass through a 35-mesh screen, and having an apparentdensity-from 41:3 to 125 grams per cubic centiinetenr neglecting theweight of HF contained therein.

2. A process for removing HF from a fluid containing. the same whichcomprises absorbing said HEin-.pelletsof-sodium fluoride at atemperature from "Gl to 200 0., said pellets being mechanicallystrongbodies of volume not less than .025 cc., and having an apparentdensity from 1.3 to 1.5 grams per cubiccentimeter, neglecting theweightofrI-IFcontained therein.

3 A"processfor removing HF from a gas contaifiing the same comprisingabsorbing said gas in ailplurality of tporousi bodies of sodium fluorideof volume not-llssthan .025 cc. each and of de'nsity-'-f1"om-13"-to 1.5,neglecting the weight of absorbed therein, said porous bodies beingmaintained in a temperature range from 85 C. to 200 0., said gas beingpassed over sufficient surface of said. porous bodies to reduce theI-IFcontent to a mere trace, and thereafter passing said gas over similarporous bodies ofisodium fluoridemaintained in a temperature range"below85" C.

E." LONG; HAROLD W; CROMER'I REFERENCES CITED The followingreferences-arev of record in the article by Roperret-alj, (April 1926)V01,,45,1pa,ges.

Chemical Abstracts} article by-Meyer et-al-.,

vol. 16; (1922)- page 206,-also in Berichte'Deutsche Chemische'Gesellschaftj vol 54.'.pages 3759-66-

